In the manufacture of flexible and rigid printed circuits, a copper clad laminate is processed through many stations to form the circuits. The laminate initially includes a plastic substrate having copper clad to one or both major surfaces thereof. For flexible printed circuits, the laminate is formed in rolls of substantial lengths, such as 550 feet, and provides a base for the manufacture of many printed circuits of the same pattern formed in successive circuit sections of the laminate. For rigid printed circuits, individual rigid boards of various sizes are each formed as the laminate and provide the base for the manufacture of printed circuits thereon.
Each flexible and rigid laminate is indexed through a punch press to form a plurality of holes and slots therethrough of different sizes and shapes. For example, depending on the code of the circuit to be made, each circuit could have as few as one hole or as many as 9999 holes in each section of the flexible laminate and up to 99,999 holes in each rigid laminate. Many of the holes are extremely small and are usually round while other larger holes may be round, square or oblong. Each hole is required in the ultimate formation of the printed circuit in order for the circuit to perform as designed. Thus, it is critically important that the required number of holes and slots be formed in the copper-clad laminate when indexed through the punch press.
On occasion, the punches of the punch press become worn or damaged, or the punches may be missing. In any case, such defective or missing punches result in a missing hole or holes in successive copper-clad laminates. If the missing holes are not detected, a plurality of copper-clad laminates could be processed through the punch press and through the printed circuit forming facilities. The occurrence of the missing holes would not become apparent until after the actual circuits have been formed and are processed through final testing. Since the holes are through-plated during the manufacturing process, it would be a practical impossibility to drill the holes in the completed circuit where missing holes are detecting during final testing. Consequently, not only would there be a loss of the laminate, there would also be a costly loss in processing time, use of equipment and personnel time.
It is also possible that more than the required number of punches could be included accidently in the punch press which would result in too many holes in each section of the laminate. This may also result in the loss of rolls or boards of laminate.
Thus, it becomes apparent that some form of inspection of the laminate is required as the successive sections exit from the punch press.
Due to (1) the rapid rate at which the laminate moves through the punch press, (2) the minuteness of many of the holes, (3) the number of holes, (4) the different sizes and shapes of the holes and (5) the length of the laminate to be examined, it is a practical impossibility to perform a visual inspection. Thus, it becomes obvious quickly that an automatic hole-detecting system must be used.
Since light can be passed through the holes and slots, it would appear that a light-sensing system operating at a rate compatible with the speed of the moving laminate would be able to count the holes. Further comparison techniques, in the case of the flexible laminate, could be employed to determine whether any of the successive sections contain less than the desired number of holes.
In consideration of a light-sensing system, many side-to-side scans of the moving laminate must be employed to insure that the smallest hole is detected and counted. Due to the minuteness of the small holes, the scan frequency of any light-sensing system used must be sufficient to provide at least one scan line to sense the light passing therethrough. Consequently, the larger holes will be scanned several times. Since the light-sensing system would typically respond and count each time light is sensed, each of the larger holes would provide a light-sensed condition each time it is scanned and thereby provide multiple counts for a single hole. Obviously, a system of this type would not satisfy the requirements of examining a laminate having holes of different shapes and sizes.
In the case of flexible laminate, the pattern of holes and slots is repeated many times along a roll or length of the laminate. If a light-sensing system is used, it must be reset at extremely short intervals to insure that it examines each circuit section with no carryover of hole count from preceding sections.
A system for counting holes in a continuous web or laminate of flexible material is described in a copending application Ser. No. 903,338, (now U.S. Pat. No. 4,205,769) filed on May 5, 1978 in the name of F. H. Blitchington and assigned to the assignee of this application. In that system, the continuous web or laminate of flexible material is indexed through a punch press whereat holes of different shapes and sizes are punched in repetitive patterns through successive sections of the laminate. The laminate is then moved beneath a diffused light source so that light passes through the holes and is sensed by a light-sensing camera which laterally and cyclically scans at a rapid rate the underside of the moving laminate.
During the rapid scanning by the camera, each hole may be scanned many times due to the size and shape of the hole. Consequently, light is sensed many times for the same hole and a corresponding number of signals are developed by the camera. A hole counting system, which receives the developed signal from the camera, includes a delay-and-compare circuit wherein each signal related to a given hole is delayed by one scan cycle and compared with the next signal related to the same hole. When the given hole has passed, the camera does not sense light on the next scanning cycle. This condition, when compared with the immediately previous light-sensed condition, results in the development of an output hole-count pulse from the delay-and-compare circuit.
The count pulses of each section of the laminate are fed to a counter-comparator circuit, and are counted and compared with a desired preset count for each section. If the actual and preset counts of a selected number of successive sections do not compare, an alarm is sounded and a signal is fed to the punch press to stop the punching operation. A detailed description of the system for counting holes in flexible material is provided in the above-mentioned pending application which, by reference thereto, is incorporated herein.
When the above-described system is used for counting holes in a laminate of rigid material, the light from the light source is further diffused as it passes through the hole due to the thickness of the rigid board, thus affecting the operation of the light-sensing camera. A light source such as a collimated laser beam could be used with a rotating polyhedral mirror and two convex lenses to deflect the beam and thereby produce successive parallel beams necessary for side-to-side scans of the moving material. However, if the faces of the rotating polyhedral mirror are not uniformily parallel to the axis of rotation, the laser beam will not trace coincident paths across the material during a complete revolution of the mirror. For example, the scan from a first mirror face which is parallel to the axis could pass light through a given hole, light from a second mirror face which is not parallel to the axis could miss the hole and light from a third mirror face which is parallel to the axis could again pass light through the hole. This would result in a hole count of two when actually there was only one hole.
U.S. Pat. No. 4,002,830, which issued to J. B. Brown et al., discloses an apparatus for compensating for optical error in a rotative mirror. Inherent defects in the angular relationship between facets of a rotating polygonal mirror used to sequentially scan a beam of radiation are corrected by an optical reflecting or refracting element pivotally mounted in the path of the radiation. With the use of electromechanical devices energized by timed electrical signals of appropriate value, the mirror or refracting elements is pivoted to correct the scanning errors caused by angular defects in the rotating mirror. The effects of variations in the angles between facets of the polygonal mirror are also corrected by an electronic circuit which includes a delay capable of delaying the scanning by a predetermined amount. Control means for the electromechanical devices and for the delay device are preprogrammed to make the proper adjustments for each facet of the rotating polygonal mirror.
Consequently, there is a need for a universal system which will count accurately the actual number of holes in both flexible and rigid types of material.